U.S. patent number 7,669,427 [Application Number 11/453,781] was granted by the patent office on 2010-03-02 for temperature calibration device driving heating/cooling modules in a manner to allow operation over wide temperature range.
This patent grant is currently assigned to Fluke Corporation. Invention is credited to Frank E. Liebmann, Richard W. Walker.
United States Patent |
7,669,427 |
Walker , et al. |
March 2, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
Temperature calibration device driving heating/cooling modules in a
manner to allow operation over wide temperature range
Abstract
A temperature calibration device uses Peltier cells for heating
and cooling. The Peltier cells are driven by a power controller
that gradually changes the power applied to the Peltier cells from
a starting power to a target power. However, during this
transition, the power controller holds the power applied to the
Peltier cells constant for a period to minimize the stress on the
Peltier cells. This period of constant period may last until the
temperature of an object reaches a predetermined temperature, or it
may have a fixed or variable duration, which may be based on the
difference between the starting power and the target power. By
changing the power applied to the Peltier cells in this manner, the
useful life of the Peltier cells is preserved.
Inventors: |
Walker; Richard W. (Alpine,
UT), Liebmann; Frank E. (American Fork, UT) |
Assignee: |
Fluke Corporation (Everett,
WA)
|
Family
ID: |
38861505 |
Appl.
No.: |
11/453,781 |
Filed: |
June 14, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070291815 A1 |
Dec 20, 2007 |
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Current U.S.
Class: |
62/3.7; 62/3.2;
62/259.2 |
Current CPC
Class: |
G01K
15/005 (20130101) |
Current International
Class: |
F25B
21/02 (20060101) |
Field of
Search: |
;62/3.2,3.3,3.7,259.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Ferrotec America Corporation, "Thermoelectric Modules Reliability
Report", Issue 1, Apr. 2001, pp. 1-12. cited by other.
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Primary Examiner: Jones; Melvin
Attorney, Agent or Firm: Dorsey & Whitney LLP
Claims
We claim:
1. A method of operating a temperature calibration device having a
plurality of Peltier cells to heat or cool an object in thermal
communication with the Peltier cells from a starting temperature to
a target temperature, the method comprising: applying a voltage to
the Peltier cells that makes the temperature of the object
substantially equal to the starting temperature; changing the
voltage applied to the Peltier cells to an intermediate voltage;
holding the voltage applied to the Peltier cells at the
intermediate voltage; and after holding the voltage applied to the
Peltier cells at the intermediate voltage, changing the voltage
applied to the Peltier cells to a voltage that makes the
temperature of the object substantially equal to the target
temperature.
2. The method of claim 1 wherein the act of holding the voltage
applied to the Peltier cells at the intermediate voltage comprises
holding the voltage applied to the Peltier cells at the
intermediate voltage until the temperature of the object reaches an
intermediate temperature.
3. The method of claim 2 wherein the intermediate temperature
comprises a temperature midway between the starting temperature and
the target temperature.
4. The method of claim 1 wherein the act of holding the voltage
applied to the Peltier cells at the intermediate voltage comprises
holding the voltage applied to the Peltier cells at the
intermediate voltage for a period of predetermined duration.
5. The method of claim 4 wherein the predetermined duration is a
function of the temperature difference between the starting
temperature and the target temperature.
6. The method of claim 4 wherein the predetermined duration is a
function of the difference between the voltage applied to the
Peltier cells to make the temperature of the object substantially
equal to the starting temperature and the voltage applied to the
Peltier cells to make the temperature of the object substantially
equal to the target temperature.
7. The method of claim 6 wherein the voltage applied to the Peltier
cells to make the temperature of the object substantially equal to
the starting temperature has a magnitude that is substantially
greater than the voltage applied to the Peltier cells to make the
temperature of the object substantially equal to the target
temperature, and wherein the intermediate voltage has the same
polarity as the voltage applied to the Peltier cells to make the
temperature of the object substantially equal to the starting
temperature.
8. The method of claim 1 wherein the intermediate voltage comprises
zero volts.
9. The method of claim 1 wherein the acts of changing the voltage
applied to the Peltier cells to an intermediate voltage and, after
holding the voltage applied to the Peltier cells at the
intermediate voltage, changing the voltage applied to the Peltier
cells to a voltage that makes the temperature of the object
substantially equal to the target temperature comprises gradually
changing the voltage applied to the Peltier cells.
10. A method of operating a temperature calibration device having a
plurality of Peltier cells to heat or cool an object in thermal
communication with the Peltier cells, the method comprising:
applying a starting power to the Peltier cells; changing the power
applied to the Peltier cells from the starting power to an
intermediate power; holding the power applied to the Peltier cells
at the intermediate power for a period of constant power; and after
holding the power applied to the Peltier cells at the intermediate
power for the period of constant power, changing the power applied
to the Peltier cells to a target power.
11. The method of claim 10 wherein the act of holding the power
applied to the Peltier cells at the intermediate power for the
period of constant power comprises holding the power applied to the
Peltier cells at the intermediate power until the temperature of
the object reaches an intermediate temperature.
12. The method of claim 11 wherein the intermediate temperature
comprises a temperature midway between a starting temperature and a
target temperature.
13. The method of claim 10 wherein the period of constant power has
a predetermined duration.
14. The method of claim 13 wherein the predetermined duration is a
function of the temperature difference between a starting
temperature and a target temperature.
15. The method of claim 13 wherein the predetermined duration is a
function of the difference between the starting power and the
target power.
16. The method of claim 15 wherein the starting power is
substantially greater than the target power, and wherein a voltage
applied to the Peltier cells at the intermediate power has the same
polarity as a voltage applied to the Peltier cells at the starting
power.
17. The method of claim 10 wherein the intermediate power comprises
zero volts.
18. The method of claim 10 wherein the act of changing the power
applied to the Peltier cells from the starting power to the
intermediate power comprises gradually changing the power applied
to the Peltier cells from the starting power to the intermediate
power.
19. The method of claim 10 wherein the act of changing the power
applied to the Peltier cells from the intermediate power to the
target power comprises gradually changing the power applied to the
Peltier cells from the intermediate power to the target power.
20. A temperature calibration device, comprising: a block of
thermally conductive material that is structured to be placed in
thermal communication with a device to be calibrated; a plurality
of Peltier cells in thermal contact with the block of thermally
conductive material; a power controller coupled to the Peltier
cells, the power controller being structured to apply power to the
Peltier cells and to change the power applied to the Peltier cells
from a starting power to an intermediate power, to hold the power
applied to the Peltier cells at the intermediate power for a period
of constant power, and to then change the power applied to the
Peltier cells to a target power; and a temperature sensor coupled
to the power controller and in thermal communication with the block
of thermally conductive material, the power controller being
structured to hold the power applied to the Peltier cells at the
intermediate power until the temperature of the object indicated by
the temperature sensor reaches an intermediate temperature, the
power controller being structured to calculate the intermediate
temperature as a temperature midway between a starting temperature
and a target temperature.
21. A temperature calibration device, comprising: a block of
thermally conductive material that is structured to be placed in
thermal communication with a device to be calibrated; a plurality
of Peltier cells in thermal contact with the block of thermally
conductive material; and a power controller coupled to the Peltier
cells, the power controller being structured to apply power to the
Peltier cells and to change the power applied to the Peltier cells
from a starting power to an intermediate power, to hold the power
applied to the Peltier cells at the intermediate power for a period
of constant power, and to then change the power applied to the
Peltier cells to a target power, wherein the power controller is
structured to hold the power applied to the Peltier cells at the
intermediate power for a period having a predetermined
duration.
22. The temperature calibration device of claim 21 wherein the
power controller is structured to determine the predetermined as a
function of the difference between the starting power and the
target power.
23. A temperature calibration device, comprising: a block of
thermally conductive material that is structured to be placed in
thermal communication with a device to be calibrated; a plurality
of Peltier cells in thermal contact with the block of thermally
conductive material; and a power controller coupled to the Peltier
cells, the power controller being structured to apply power to the
Peltier cells and to change the power applied to the Peltier cells
from a starting power to an intermediate power, to hold the power
applied to the Peltier cells at the intermediate power for a period
of constant power, and to then change the power applied to the
Peltier cells to a target power, wherein the power controller is
structured to gradually change the power applied to the Peltier
cells from the starting power to the intermediate power.
24. A temperature calibration device, comprising: a block of
thermally conductive material that is structured to be placed in
thermal communication with a device to be calibrated; a plurality
of Peltier cells in thermal contact with the block of thermally
conductive material; and a power controller coupled to the Peltier
cells, the power controller being structured to apply power to the
Peltier cells and to change the power applied to the Peltier cells
from a starting power to an intermediate power, to hold the power
applied to the Peltier cells at the intermediate power for a period
of constant power, and to then change the power applied to the
Peltier cells to a target power, wherein the power controller is
structured to gradually change the power applied to the Peltier
cells from the intermediate power to the target power.
25. A temperature calibration device, comprising: a block of
thermally conductive material that is structured to be placed in
thermal communication with a device to be calibrated; a plurality
of Peltier cells in thermal contact with the block of thermally
conductive material; and a power controller coupled to the Peltier
cells, the power controller being structured to apply power to the
Peltier cells and to change the power applied to the Peltier cells
from a starting power to an intermediate power, to hold the power
applied to the Peltier cells at the intermediate power for a period
of constant power, and to then change the power applied to the
Peltier cells to a target power, wherein the plurality of Peltier
cells a placed on each other in at least one stack Peltier cells
having an inner Peltier cell placed on the block of thermally
conductive material, a middle Peltier cell placed on the inner
Peltier cell, and an outer Peltier cell placed on the middle
Peltier cell.
Description
TECHNICAL FIELD
This invention relates to electrically powered devices, and, more
particularly, to temperature calibration devices using Peltier
cells to provide heating and cooling.
BACKGROUND OF THE INVENTION
A wide variety of electrically powered heating devices are in
existence to provide a wide variety of functions. For example,
temperature calibration devices, known as dry-well calibrators, are
commonly used in industry to calibrate precision temperature
probes.
Conventional dry-well calibrators use Peltier heating/cooling
modules generally containing Peltier cells to heat or cool the
temperature probes to temperatures that can be set by a user.
Electrical current having one polarity is driven through the
Peltier cells to cause the temperature of the first substrate to
rise relative to the temperature of the second substrate, thereby
heating the temperature probe being calibrated. Electrical current
having the opposite polarity causes the temperature of the first
substrate to fall relative to the temperature of the second
substrate, thereby cooling the temperature probe being calibrated.
A current is produced by applying an electrical potential across a
Peltier cell. A Peltier cell has a specified maximum current.
Operating a Peltier cell near or above its maximum current can
cause premature performance degradation or failure. Therefore, a
limiting factor in the operating range of a dry-well calibrator is
the maximum current of the Peltier cells used in the dry-well
calibrator. To maximize the operating range of dry-well
calibrators, the Peltier cells are frequently driven as close as
possible to their maximum current without substantially shortening
their lifetime. The current that flows depends on the potential,
the resistance of the Peltier cell, and the temperature
differential across the Peltier cell. Applying a voltage to a
Peltier cell while it has an opposing temperature differential can
result in excessive current through the Peltier cell. Specifically,
if Peltier cells are still cold when the polarity of power is
abruptly switching to induce heating, the cold cells will produce a
voltage that effectively increases the heating voltage applied to
the cells, thereby potentially inducing excessive current. The same
phenomena can occur when quickly transitioning from heating to
cooling. It is often necessary to quickly transition dry-well
calibrators from heating to cooling, or vice-versa. Sometimes this
transition is from full maximum heating to full maximum cooling.
During this transition time, until the temperature differentials
settle, it is possible for the Peltier cells to temporarily
experience excessive current unless means are taken to prevent
it.
Peltier cells used in dry-well calibrators are usually stacked on
top of each other to provide heating and cooling over a range of
temperatures that is wider than this temperature differential of
each cell. The total temperature differential of a heating/cooling
system is substantially equal to the sum of the temperature
differentials that can be developed across all of the stacked
Peltier cells. The temperature differential that can be developed
between the substrates of each Peltier cell is limited to a
specified maximum temperature. Operating a Peltier cell near or
above its maximum temperature can cause premature performance
degradation or failure. Therefore, a limiting factor in the
operating range of a dry-well calibrator is the maximum specified
temperature differential of the Peltier cells used in the dry-well
calibrator. To maximize the operating range of dry-well
calibrators, the Peltier cells are frequently driven as close as
possible to their maximum temperature differentials without
substantially shortening their lifetime. During a transition from
heating to cooling or vice-versa, until the temperature
differentials settle, it is possible for some of the Peltier cells
in the stack to temporarily experience excessive temperature
differentials unless means are taken to prevent it.
Operation of the Peltier cells near or above their maximum
specifications can severely limit the useful lives of the cells.
Frequent replacement of the Peltier cells can be very expensive,
not only because of the cost of the cells, but also because of the
cost of labor required to disassemble dry-well calibrators to
replace the cells and the downtime costs during such replacement.
As a result, there is an inevitable tradeoff between achieving a
wide operating range and achieving reliable performance. Even where
the Peltier cells are not frequently driven to high maximum
temperature differentials, their useful lifetimes can be
unreasonably short. It has been recognized that this shortening in
the useful life of the Peltier cells can be caused by abruptly
changing between heating and cooling functions, particularly
between full maximum heating and full maximum cooling. However,
despite procedures to simply limit the rate at which the applied
potential changes when transitioning from heating to cooling and
vice-versa, it is still possible for the maximum current and
temperature differential of the Peltier cells to be temporarily
exceeded and the useful life of Peltier cells can be unduly
limited.
There is therefore a need for a dry-well calibrator using Peltier
cells that applies power to the Peltier cells in a manner that does
not unduly shorten their useful life.
SUMMARY OF THE INVENTION
A temperature calibration device includes a block of thermally
conductive material that is placed in thermal communication with a
device to be calibrated. The block is in thermal contact with a
plurality of Peltier cells that receive power from a power
controller. The power controller is structured to intelligently
limit the magnitude and polarity of the electrical potential
applied to the Peltier cells during transition from one temperature
to another. The result is that the starting potential applied to
the Peltier cells from a starting temperature changes to one or
more or continuous intermediate levels of potential while in
transition before the final level of potential is applied to
produce the target temperature. The magnitudes and polarities of
the intermediate potentials and the duration at each potential may
be determined by a variety of techniques. For example, the
magnitude and polarity of the potential may be a continuous
function of the temperature of the block, or a period of constant
intermediate potential may last until the block reaches a certain
temperature, or an intermediate potential may be of predetermined
duration. The intermediate potentials may also depend on other
conditions, such as the difference between the starting power and
the target power. After the period of intermediate power, the power
controller is structured to change the power applied to the Peltier
cells to a target power. As a result of this scheme, during
transition from one temperature to another, the limited power
prevents excessive temperature differentials from developing and
also prevents excessive current through the Peltier cells.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 an exploded isometric view of some of the internal
components of a dry-well calibrator according to one example of the
invention.
FIG. 2 is a cross-sectional view of the internal components of the
dry-well calibrator shown in FIG. 1.
FIG. 3 is an exploded isometric view of a case surrounding the
internal components of the dry-well calibrator shown in FIG. 1.
FIG. 4 is a front elevational view of the dry-well calibrator of
FIG. 1.
FIGS. 5A-C are graphs showing the manner in which power is applied
to the Peltier cells in the dry-well calibrator of FIGS. 1-4
according to one example of the invention.
FIG. 6 is a block diagram of a system for driving Peltier cells in
the dry-well calibrator of FIGS. 1-4 according to one example of
the invention.
DETAILED DESCRIPTION
Embodiments of the present invention are directed to a system and
method for allowing a dry-well calibrator to operate over a wide
range of temperatures without adversely affective the service life
of Peltier cells used in the dry-well calibrator. Certain details
are set forth below to provide a sufficient understanding of the
invention. However, it will be clear to one skilled in the art that
the invention may be practiced without these particular details. In
other instances, well-known circuits, control signals, and timing
protocols have not been shown in detail in order to avoid
unnecessarily obscuring the invention.
The internal components of a heating block assembly for a typical
dry-well calibrator 10 are shown in FIG. 1. The dry-well calibrator
10 includes a cylindrical insert 14 having one or more cylindrical
bores 16a,b,c sized to receive temperature probes "P" having
corresponding dimensions. The insert 14 is typically manufactured
from a thermally conductive metal. The insert 14 fits into a
cylindrical bore 18 formed in a heated block 20 of a suitable
material, such as a metal with good thermal conduction properties.
The block 20 has a configuration that is rectangular in both
vertical and horizontal cross-section, although, of course, it may
also have a square, round or other configuration. The inside
diameter of the bore 18 is only slightly larger than the outside
diameter of the insert 14 to ensure good heat conduction from the
block 20 to the insert 14.
With reference also to FIG. 2, a pair of upper
Peltierheating/cooling modules 30, 32 and a pair of lower Peltier
heating/cooling modules 36, 38 are bonded to opposite surfaces of
the block 20. Each of the Peltier heating/cooling modules 30-38
includes a first Peltier cell 40 having an inner substrate 42 (FIG.
1) bonded to the block 20. A second Peltier cell 44 has an inner
substrate 46 (FIG. 1) that is bonded to an outer substrate 48 (FIG.
1) of the first cell 40. Temperature conductive plates 50 are
bonded to outer substrates 54 (FIG. 1) of the second cells 44. A
pair of Peltier cells 60, 62 each having inner and outer substrates
66, 68, respectively, (FIG. 1) have their inner substrates 66
bonded to an outer surface of the plates 50. The Peltier cells 60,
62 are positioned so that their abutting edges overlie the centers
of the first and second Peltier cells 40, 44. Finally, conductive
leads (not shown) supply electrical power to the Peltier cells 40,
44, 60, 62. As is well-known in the art, electrical power having
one polarity causes the temperature of the inner substrates to rise
relative to the temperature of the outer substrates thereby heating
the block 20. Electrical power having the opposite polarity causes
the temperature of the inner substrates to fall relative to the
temperature of the outer substrates, thereby cooling the block 20.
When the Peltier cells 40, 44, 60, 62 are used for either heating
or cooling, the resulting temperature changes imparted to the outer
surfaces 68 of the Peltier cells are moderated by heat sinks 74
abutting the outer substrates 68 (FIG. 1) of the cells 60, 62.
With reference also to FIG. 3, the above-described components of
the dry-well calibrator 10 are surrounded by an outer case 80
formed by case sections 80a,b,c,d. A fan board 84 containing a fan
86 is positioned inside the case section 80a so that the fan 86 is
behind a grill 88. The case 80 is separated from the heat sinks 74
by an insulating space, and the fan 86 provides airflow through
this insulating space to remove heat from or supply heat to the
heat sinks 74.
As best shown in FIG. 4, a keypad 90 mounted on a panel 92 of the
case section 80a is connected to the control circuitry (FIG. 6) to
control the operation of the dry-well calibrator 10. A display 94,
which is also connected to the control circuitry provides
information about the operation of the dry-well calibrator 10, such
as the temperature of the block 20.
In operation, the keypad 90 (FIG. 4) is used to set the temperature
of the block 20 as well as the rate at which the temperature of the
block 20 is changed to reach the set temperature. If the
temperature set by the keypad 90 is for a temperature above ambient
temperature, power having a first polarity is applied to wires that
are connected to the Peltier cells 40, 44, 60, 62, thereby causing
the cells to cool the block 20. If the temperature set by the
keypad 90 is for a temperature below ambient temperature, power
having a first polarity is applied to wires that are connected to
the Peltier cells 40, 44, 60, 62 to cause the cells to cool the
block 20. Once the temperature of the block 20 has stabilized, the
temperature probe P (FIG. 1) is inserted into a corresponding sized
bore 16 of the insert 14. The probe P is then calibrated by
ensuring that a readout device (not shown) connected to the probe P
indicates the temperature of the probe P is equal to the set
temperature of the dry-well calibrator 10.
As explained above, the useful life of the Peltier cells 40, 44,
60, 62 can be drastically reduced by operating them at or near
their maximum specified temperature differentials and by abruptly
changing them between full heating power and full cooling power. To
minimize this damage, it is known to keep the Peltier cells 40, 44,
60, 62 at lower temperature differentials and avoid abrupt changes
in current through the Peltier cells 40, 44, 60, 62. It has been
discovered that Peltier cell damage can also be reduced by keeping
the power applied to the Peltier cells 40, 44, 60, 62 at or close
to zero for a short period when transitioning from heating to
cooling and vice-versa.
The power level of this constant heating or cooling power ideally
depends on the starting power from which the calibrator 10 is
transitioning and a target power to which the calibrator 10 is
transitioning. For example, as shown in FIG. 5A, if the dry-well
calibrator 10 is transitioning from a very high heating power to a
very high cooling power, the period of constant power level may be
at a level in which no power is being applied to the Peltier cells
40, 44, 60, 62. In one embodiment of the invention, the power
remains at the constant power level for a predetermined period of
time. However, the duration "T" of this predetermined period of
time may be a function of the difference between the starting power
and the target power. In another embodiment of the invention, the
power remains at the constant power level until the temperature of
the block 20 reaches a predetermined temperature. For example, this
predetermined temperature can be midway between the starting
temperature and the target temperature.
Another example is shown in FIG. 5B. In this example, the starting
power is a moderately high heating power and the target power is a
very high cooling power. The period of constant power during this
transition is therefore at a power that causes the Peltier cells
40, 44, 60, 66 to cool the dry-well calibrator 10.
A final example is shown in FIG. 5C in which the starting power is
a slight heating power and the target power is a slight cooling
power. In such case, relatively little stress is imposed on the
Peltier cells 40, 44, 60, 66, particularly if, as shown in FIG. 5C,
the power is gradually changed from a heating level to a cooling
level. Therefore, it is not necessary to maintain the power to the
Peltier cells 40, 44, 60, 66 constant for a period of time when
transitioning from heating to cooling.
A control system 100 for driving the Peltier cells 40, 44, 60, 66
according to one example of the invention is shown in FIG. 6. The
control system 100 includes a temperature sensor 104 mounted on a
surface to be monitored, such as the block 20 (FIGS. 1-3). The
temperature sensor 104 provides an analog signal indicative of the
temperature of the block 20. This analog signal is applied to an
analog-to-digital ("A/D") converter 106, which outputs a plurality
of bits on a bus 108 indicative of the temperature of the block 20.
These bits are applied to a controller 110, which may be
implemented by conventional means, such as a properly programmed
microprocessor. The controller 110 executes a program stored in a
read-only memory ("ROM") 112, which is connected to the controller
110 by address, control and data buses 114. The controller 110
receives user commands from the keypad 90 (FIG. 4) and applies
signals to the display 94 for providing information to the user, as
explained above. The controller 110 also outputs a plurality of
bits on a bus 116 to a digital-to-analog ("D/A") converter 118. The
D/A converter 118 outputs a corresponding positive or negative
analog signal to a power driver 120, which, outputs corresponding
voltage to the Peltier cells 40, 44, 60, 66. As previously
explained, the polarity of the voltage determines whether the
Peltier cells 40, 44, 60, 62 will heat the block 20 or cool the
block 20, and the magnitude of the voltage determines the heating
or cooling power.
In operation, it is assumed that the control system 100 is
operating in a heating mode to regulate the temperature of the
block 20 at a preset value using feedback applied to the controller
110 from the temperature sensor 104 through the A/D converter 106.
The user then enters a command through the keypad 90 to set the
target temperature to which the block 20 should be heated or
cooled. Based on this starting temperature and the target
temperature set by the keypad 90, the controller 110 determines a
timed power schedule at which power will be applied to the Peltier
cells 40, 44, 60, 66. This power schedule includes not only the
slew rate, i.e., the rate at which the voltage from the power
driver 120 will change, but also a target power level and the
intermediate power level that will be applied during the constant
power period. In the control system 100 shown in FIG. 6, when the
voltage output from the power driver 120 reaches a voltage
corresponding to the constant, intermediate power level, the
voltage remains constant until the temperature of the block 20
reaches a predetermined temperature. In the system 100, this
predetermined temperature is midway between the starting
temperature and the target temperature. However, in other
embodiments of the invention, the predetermined temperature may be
determined in another manner, and the duration of the constant
power period may be of fixed duration or it may be determined by
other means. For example, as mentioned above, the duration of this
period of constant power may be a function of the difference
between the starting power or temperature and the target power or
temperature.
By applying power to the Peltier cells 40, 44, 60, 66 in a manner
that maintains the power constant for a period during transitions
between heating and cooling, the dry-well calibrator 10 can operate
over a wide temperature range without unduly limiting the useful
life of the Peltier cells or otherwise degrading their
performance.
Although the present invention has been described with reference to
the disclosed embodiments, persons skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention. Such
modifications are well within the skill of those ordinarily skilled
in the art. Accordingly, the invention is not limited except as by
the appended claims.
* * * * *